Fluoropolymers are generally made in an aqueous media, which provides a sufficient heat sink for managing the heat of polymerization.
In order to achieve a stable dispersion or emulsion in aqueous media, a suitable surfactant or emulsifier must be employed to obtain a stable dispersion having a high yield and high molecular weight. The copolymerization of 2,3,3,3 tetrafluoropropene with vinylidene fluoride is described in U.S. Pat. No. 2,970,988, U.S. Pat. No. 3,085,996, U.S. Pat. No. 6,818,258, U.S. Pat. No. 7,803,890, US 2008153977 and WO10005757. Fluorinated-surfactants are used in forming these copolymers because they can yield stable latex and high molecular weight fluoropolymers. The fluorinated-surfactants typically used in emulsion polymerization of 2,3,3,3-tetrafluoropropene-based-polymers, such as the ammonium salt of perfluoro octanoic acid or salts of perfluoro alkyl sulfonic acids, are expensive. They also present an environmental concern related to bio-persistence and EPA is restricting their uses.
It is therefore desirable to produce copolymers of 2,3,3,3 tetrafluoropropene, without the use of fluorosurfactants without compromising the properties of the resultant fluoropolymers.
Attempts have been made to find a suitable emulsifier in place of fluorinated surfactant for polymerization of vinylidene fluoride, vinyl fluoride, trifluoroethylene, tetrafluoroethylene (TFE), chlorothrifluoroethylene (CTFE) and hexafluoropropylene (HFP) and their respected copolymers. For example, U.S. Pat. No. 6,512,063 discloses the use of the sodium salt of organic sulfonates as the non-fluorinated surfactant. WO 2007018783 formation of fluoropolymers using non-fluorinated surfactants selected from the group consisting of alkyl phosphonic acids, polyphosphonic acid, polyacrylic acids, polyvinyl sulfonic acid, and salts was described. Other references describing the use of non-fluorinated surfactants for producing fluoropolymers and fluoroelastomers include WO 02/088207, and U.S. Application 2006/0135716. None of the references disclose the polymerization of 2,3,3,3-tetrafluoropropene-based-polymers using non-fluorinated surfactants. U.S. Pat. No. 4,128,517 discloses a cleaning method for fluoropolymer dispersions produced using fluorinated-surfactants where dispersion is subjected to industrial post-treatment processes, such as coagulation, washing and drying.
Surprisingly, fluoropolymers made in accordance with the present invention can be cleaned from contaminants produced during dispersion polymerization even though not made with fluorinated surfactant. U.S. Pat. No. 6,794,550 describes a process in which fluoropolymer dispersions were synthesized in the presence of fluorinated emulsifiers. Nonionic emulsifiers were post-added to the dispersions, and then a portion of fluorinated surfactant was removed by means of steam-volatilization at low pH. The disclosed processes could never remove all of the fluorinated surfactant; therefore, the resultant fluoropolymer dispersion is not absolutely free of fluorinated surfactant, and a portion of the fluorinated surfactant will remain in the final dispersion. Moreover, the shelf-stability of said dispersion would be considerably reduced if not totally diminished due to heating the dispersion up to the steaming point at low pH. Further, the use of fluorosurfactants in the process, even when latter removed creates a waste stream containing fluorosurfactants and the associated environmental issues.
Surprisingly it has now been found that fluoro-copolymer of 2,3,3,3-tetrafluoropropene, and especially copolymers with vinylidene fluoride can be synthesized over a wide ratio of comonomers, to produce stable emulsions of high molecular weight copolymers. The copolymers formed having excellent physical properties, including a high degree of polymerization, high melt viscosity, and high melting temperature. Surprisingly the molecular weight, as seen in the melt viscosity, is much higher than that found in similar copolymers made with fluorosurfactants. Excellent cleanliness of reactor surfaces after polymerization is complete was also noted. Additionally the copolymer has good whiteness, and retains the whiteness on aging.